For example, a radar apparatus is disclosed in US 2005/0200833 (JP 2005-257405A). According to this radar apparatus, a laser beam is irradiated from a laser diode according to a drive signal from a laser radar CPU, and a reflective light corresponding to the laser beam is received by a light receiving element. The light receiving element outputs a voltage signal (light receiving signal) corresponding to the intensity of the received reflective light, and converts the outputted light receiving signal to a digital signal by means of an A/D conversion section which samples and A/D-converts the voltage signal. In a timer section following the A/D conversion section, an integration signal is obtained by integrating the light receiving signals which have been converted into the digital signals, and time of generation of a peak value of the peak waveform which is obtained from a noise reference value is obtained. In this example, when the time of generation of the peak value is obtained, a peak center estimate (period estimate) is conducted as follows.
As shown in FIG. 12, the maximum intensity of the A/D conversion results at a sampling point of the light receiving signal (time point when sampling is conducted by the A/D conversion) is detected, and the maximum intensity is multiplied by a given coefficient k (k is a coefficient that is equal to or lower than 1, for example, 0.5) as represented by the following expression to set a threshold value Ith.Threshold value Ith=(Maximum intensity)×k(0<k<1)  (Ex. 1)
When the threshold value is set by calculation of EX. 1, the threshold value is compared with the intensities of the A/D conversion results at each of the sampling points. With the comparison, the A/D conversion results of two points between which the threshold value is interposed are detected at two portions of a leading portion and a trailing portion of the light receiving signal waveform. As a result, as shown in FIG. 13, two successive points (t1, a1) and (t2, a2) are detected between which the threshold value Ith lies at the leading or rising portion of the light receiving signal waveform. Likewise, two points (t3, a3) and (t4, a4) are detected between which the threshold value Ith lies at the trailing or falling portion of the light receiving signal waveform. Reference ti (i=1, 2, 3, 4) represents a time from the light emission start at the sampling points, and ai (i=1, 2, 3, 4) represents the A/D conversion results at the sampling points.
Then, a leading time T1 and a trailing time T2 when it is assumed that the waveform of the received signal crosses the threshold value Ith are calculated by the aid of the four detected points. More specifically, the leading time T1 and the trailing time T2 are found by calculating the expression of linear interpolation represented by the following expression.T1=(Ith−a1)×(t2−t1)/(a2−a1)+t1  (Ex.2)T2=(a3−Ith)×(t4−t3)/(a3−a4)+t3  (Ex.3)
The time of the peak center is estimated by calculating the following expression on the basis of the calculation results of the leading time T1 and the trailing time T2.Peak center estimated time=(T1+T2)/2  (Ex. 4)
When the peak center estimated time is obtained by calculating the above Expression 4, a time difference Δt between a laser beam emission start time and a peak center estimated time is calculated. Thereafter, a distance to the reflective object is calculated by calculating the following expression on the basis of the time difference Δt that is outputted from a time measurement section in the laser radar CPU.
                                                                        Distance                ⁢                                                                  [                m                ]                            =                            ⁢                              Light                ⁢                                                                  ⁢                                  speed                  ⁡                                      [                                          m                      ⁢                                              /                                            ⁢                      ns                                        ]                                                  ×                Δ                ⁢                                                                  ⁢                                                      t                    ⁡                                          [                      ns                      ]                                                        /                  2                                                                                                        =                            ⁢                                                0.15                  ⁢                                                                          [                                      m                    ⁢                                          /                                        ⁢                    ns                                    ]                                ×                Δ                ⁢                                                                  ⁢                                  t                  ⁡                                      [                    ns                    ]                                                                                                          (                  Ex          .                                          ⁢          5                )            
This distance calculation still includes some errors and hence need be improved to be more accurate.